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120 Gb/s Multi-Channel THz Wireless Transmission and THz Receiver Performance
Analysis
Jia, Shi; Yu, Xianbin; Hu, Hao; Yu, Jinlong; Morioka, Toshio; Jepsen, Peter Uhd; Oxenløwe, Leif Katsuo
Published in:
IEEE photonic Technology Letters
Link to article, DOI:
10.1109/LPT.2016.2647280
Publication date:
2017
Document Version
Peer reviewed version
Link back to DTU Orbit
Citation (APA):
Jia, S., Yu, X., Hu, H., Yu, J., Morioka, T., Jepsen, P. U., & Oxenløwe, L. K. (2017). 120 Gb/s Multi-Channel THz
Wireless Transmission and THz Receiver Performance Analysis. IEEE photonic Technology Letters, 29(3), 310-
13. https://doi.org/10.1109/LPT.2016.2647280
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1
Abstract—A photonic multi-channel terahertz (THz) wireless
transmission system in the 350-475 GHz band is experimentally
demonstrated. The employment of six THz carriers modulated
with 10 Gbaud Nyquist quadrature phase-shift keying (QPSK)
baseband signal per carrier results in an overall capacity of up to
120 Gbit/s. The THz carriers with high-frequency stability and
low phase noise, are generated based on photonic photo-mixing of
25 GHz spaced six optical tones and a single optical local oscillator
(LO) derived from a same optical frequency comb (OFC) in an
ultra-broadband uni-travelling carrier photodiode (UTC-PD).
The bit-error-rate (BER) performance below the hard decision
forward error correction (HD-FEC) threshold of 3.8×10
-3
for all
the channels is successfully achieved after wireless delivery.
Furthermore, we also investigate the influence of the harmonic
spurs in a THz receiver on the performance of transmission
system, and the experimental results suggest more than 30 dB
spur suppression ratio (SSR) in down-converted intermediate
frequency (IF) signals for obtaining less than 1 dB interference.
Index Terms—THz photonics, THz wireless communication,
uni-travelling carrier photodiode (UTC-PD).
I. INTRODUCTION
CCORDING to the Edholm’s law of bandwidth, the demand
of bandwidth to serve both of the increasing wireline and
wireless communication data rates is growing every year [1]，
as represented in Fig. 1. The optical fiber-wireless seamless
networks, where data signals are delivered through optical fiber
cables and air without any changes to the modulation formats
and data rates, will serve as the key building block to support
the next generation network for the bandwidth-hungry
applications [2]. However, Fig. 1 illustrates that wireless links
require further development to keep up with the capacity in the
This work was supported by the Chinese Scholarship Council (CSC), the
ERC-PoC project TWIST, and the Danish center of excellence CoE SPOC, and
the National Natural Science Foundation of China under Grant 61427817 and
Grant 61405142.
S. Jia is with the Lab of Fiber-Optic Communication, School of Electrical
and Information Engineering, Tianjin University, Tianjin 300072, China, and
also with the DTU Fotonik, Technical University of Denmark, DK-2800, Kgs.
Lyngby, Denmark (tjujession@tju.edu.cn).
X. Yu is with the College of Information Science and Electronic Engineering,
Zhejiang University, Hangzhou 310027, China (corresponding author,
e-mail: xyu@zju.edu.cn).
H. Hu, is with DTU Fotonik, Technical University of Denmark, DK-2800,
Kgs. Lyngby, Denmark.
J. Yu is with the Lab of Fiber-Optic Communication, School of Electrical
and Information Engineering, Tianjin University, Tianjin 300072, China.
T. Morioka, P. U. Jepsen and L. K. Oxenløwe are with DTU Fotonik,
Technical University of Denmark, DK-2800, Kgs. Lyngby, Denmark.
Fig. 1 The increase of data rates for wireless, nomadic and wireline
communication technologies against time [1].
optical fiber communication parts, in order to realize seamless
integration [3]-[6].
Since nowadays almost all the radio bands in the micro-wave
and millimeter-wave regions below 60 GHz have been
saturated, the wireless delivery over higher frequency bands
has attracted great interest in recent years [7]-[10]. Several
systems based on free-running lasers have been reported below
300 GHz frequency bands, such as 75 Gbit/s multi-channel
data transmission at 200 GHz [11] and 8 Gbit/s wireless
transmission at 250 GHz [12]. In these approaches, since the
frequency spacing of two lasers and their phases fluctuate
independently, phase noise correction process at reception is
needed in the digital signal processing (DSP), which
consequently increases the processing time and complexity. On
the other hand, optical frequency comb (OFC)-based carrier
generation has been investigated in a 100 Gbit/s multi-carrier
200 GHz wireless transmission system [13] and a 100 Gbit/s
photonic wireless transmission system operating at 237 GHz
[14]. To accommodate the target of well beyond 100 Gbit/s,
eventually Tbit/s data rates, frequency bands above 300 GHz
are considered to be able to promise an unprecedented capacity,
since an extremely large unregulated bandwidth is available in
the THz region (300 GHz-10 THz) [15]-[19]. Up to date, the
fastest reported wireless system in the THz frequency range
(above 300 GHz) is 60 Gbit/s quadrature phase shift keying
(QPSK) wireless transmission with real-time capable detection
in 400 GHz band [15]. However, the influence of THz receiver
imperfect response on the performance of overall system has
not yet analyzed in the demonstrations mentioned above.
120 Gbit/s Multi-channel THz Wireless
Transmission and THz Receiver Performance
Analysis
Shi Jia, XianbinYu*, Senior Member, IEEE, Hao Hu, Jinlong Yu, Toshio Morioka, Fellow, OSA, Peter
U. Jepsen, and Leif K. Oxenløwe
A
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2
Fig. 2 Experimental setup of multi-channel THz communication system. PM: phase modulator, ODL: optical delay line, EDFA: Erbium-doped fiber amplifier,
WSS: wavelength selectable switch, PC: polarization controller, AWG: arbitrary waveform generator, IQ Mod: in-phase and quadrature modulator, OBFP: optical
band pass filter, LO: local oscillator, Pol: Polarizer, Att.: attenuator, UTC-PD: uni-travelling carrier photodiode. (a) Optical spectrum of generated frequency comb.
(b) The combined spectrum of optical tones launching into the UTC-PD for photo-mixing generation of THz signals.
In this letter, a multi-channel THz wireless communication
system in the 350-475 GHz band is proposed and
experimentally demonstrated, with frequency division
multiplexed six carriers modulated with 10-Gbaud Nyquist
QPSK baseband signal per carrier, reaching an overall capacity
up to 120 Gbit/s. The THz carriers with high-frequency
stability and low phase noise, are generated by photonic
heterodyne mixing of optical coherent wavelengths from an
OFC. We use an ultra-broadband uni-travelling carrier
photodiode (UTC-PD) as the photo-mixing emitter and a
Schottky mixer as the electrical receiver to explore large
bandwidth in the THz region for enabling such a high capacity.
The bit error rate (BER) of the QPSK signal in each channel
after wireless delivery is below the hard decision forward error
correction (HD-FEC) limit threshold of 3.8×10
-3
. Furthermore,
the influence of harmonic spurs in the THz receiver on the
system performance is experimentally analyzed.
II. EXPERIMENTAL SETUP
As shown in Fig. 2, the experimental configuration is mainly
composed of 3 sections. Section І illustrates generation of an
optical frequency comb. Section ІІ presents optical modulation
with Nyquist QPSK signals and phase de-correlation
compensation between a single-tone optical local oscillator
(LO) and 6 optical tones with modulation. Section ІІІ describes
the THz link, basically consisting of a UTC-PD as the
photo-mixer at transmitter, a 0.5 m THz free-space link and a
Schottky mixer as the electrical receiver. Firstly, a 1550 nm
continuous wave (CW) laser with a linewidth of 100 kHz, is
employed to coherently generate the OFC by simultaneously
modulating an amplified 25 GHz sinusoidal radio frequency
(RF) signal at two cascaded phase modulators (PMs). Here, an
optical delay line (ODL) is used in-between to optimize the
delay and to achieve timing match of the two PMs, in order to
broaden the optical spectrum of the OFC for the desired THz
frequency signals generation in the 350-475 GHz band.
After an Erbium-doped fiber amplifier (EDFA-1), a
wavelength selective switch (WSS-1, Finisar 4000S) is used to
select two groups of comb lines spaced at 375-450 GHz and
output them from two different ports. One optical tone from one
port is used as the optical LO for heterodyne generation of THz
signals. A group of six equalized optical tones from the other
port is fed into an in-phase (I) and quadrature (Q) modulator,
and modulated with a 10-Gbaud Nyquist-QPSK baseband data
generated from an arbitrary waveform generator (AWG). After
optical modulation and amplification by the EDFA-2, these six
optical tones are selectively separated into the even- and
odd-order channels by the WSS-2. Here an additional 1 m fiber
is placed in the even-order channel path, in order to de-correlate
adjacent channels. Then the even- and odd-order channels are
combined together with the un-modulated optical LO via two
optical couplers. It is noted that a length-matched fiber is added
in the optical LO path, in order to compensate the optical phase
de-correlation caused by the path difference between the LO
tone and the six modulation tones [20]. The path difference is
accurately compensated when the lowest phase noise is
obtained for the photo-mixed analogue THz carriers [21]. The
combined optical signals are then amplified with the EDFA-3
and filtered with a 9 nm optical band-pass filter (OBPF) to
reject out-of-band amplified spontaneous emission (ASE).
In the photo-mixing process before the THz link, the beating
of the optical LO with the modulated optical tones in a bow-tie
antenna integrated UTC-PD [7] generates and radiates
multi-channel THz signals in the 350-475 GHz band, also
spaced by 25 GHz. Here the incident power of the optical LO is
equal to the total power of the six modulated channels, and
meanwhile a polarization controller (PC) and a polarizer are
used before the UTC-PD, to maximize the polarization
dependent optoelectronic conversion efficiency.
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The optical power launched into the UTC-PD is controlled by a
variable optical attenuator (VOA). Within the 50 cm wireless
transmission path, a pair of THz lenses is employed to collimate
the THz beam. Subsequently, the received multi-channel THz
signals after wireless propagation are individually
down-converted into microwave intermediate frequency (IF)
for demodulation by using a Schottky mixer followed by a
12-order harmonic electronic multiplier. In order to
down-convert the THz signals into the 40 GHz IF domain, the
RF LO is in the 28.6-40 GHz range. In our experiment, the RF
LO is derived either directly from a 40 GHz synthesizer, or
from a 10 GHz synthesizer with 4-time frequency
multiplication. Finally the down-converted IF signal is
amplified by two cascaded RF amplifiers with total gain of
42 dB, and then sent to a broadband sampling oscilloscope
(Keysight DSOZ634A Infiniium) for implementing
analog-to-digital conversion, as well as offline second
heterodyne down-conversion/data demodulation in the digital
domain. The sampling rate and bandwidth of the oscilloscope
are160 GSample/s and 63 GHz, respectively.
III. RESULTS AND DISCUSSIONS
The spectrum of the generated 25 GHz spaced OFC at point (a)
is shown in Fig. 2(a), where the tones labelled by blue arrows
correspond to the desired optical LO and 6 lines for optical
modulation. As illustrated in Fig. 2(b), the combined spectrum
at point (b) consists of one un-modulated LO tone and six
wavelength division multiplexing (WDM) tones. Each of six
tones is modulated with 10-Gbaud Nyquist-QPSK baseband
signals, resulting in an overall capacity of 120 Gbit/s. These 7
tones are launched into the UTC-PD for photo-mixing
generation of the 6-channel THz signals at 350 GHz, 375 GHz,
400 GHz, 425 GHz, 450 GHz and 475 GHz, respectively. The
combined 6-channel electrical spectrum is shown in Fig. 3(a),
which is investigated by measuring the down-converted IF
spectrum after wireless delivery. The blue circles in Fig. 3(a)
indicate the trends of the received electrical power and
signal-noise-ratio (SNR) variation among channels, which is
mainly caused by the frequency response of the UTC-PD,
wireless channel as well as the THz receiver. Therefore, the
frequency response of the whole THz link is also reflected in
the measured electrical spectrum. We can observe that the
400 GHz and 425 GHz channels suffer the least link loss
among them, 350- and 475 GHz the most, and 375- and
450 GHz in-between. Here, the arrangement of equal channel
spacing is to avoid the spectral overlapping after electrical
down-conversion. The spectral efficiency can be further
increased by using more advanced modulation formats (e.g.
16-QAM) and reducing the number of the guard bands, for
example by grouping channels in pairs. Moreover, the
employment of the path-length matching fiber results in high
frequency stability and low phase noise of the generated THz
carriers [20], which benefits the reception.
We have also measured the bit-error-rate (BER) for
analyzing the THz receiver performance when driving the THz
mixer with and without harmonic spurs. The comparison is
realized by using either a 40 GHz high frequency synthesizer as
Fig. 3 (a) The combined electrical spectrum of the received multi-channel THz
signals with modulation at reception. (b) The BER performance for 6 channels
after 0.5 cm wireless delivery without harmonic spurs in the THz receiver.
(c)The BER performance for 6 channels after 0.5 cm wireless delivery with
harmonic spurs in the THz receiver. (d) The power penalties between (b) and (c)
when considering the BER of 3.8×10
-3
for each channel, and the spur
suppression ratio for IF signals in each channel in the THz receiver.
the electrical LO (no spur), or a 10 GHz synthesizer combining
the 4-time frequency multiplier as the electrical LO (with spurs)
for the mixer. When no spur exists in the receiver, the measured
BER performance for all the channels is shown in Fig. 3(b),
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where two constellations corresponding to the BER of 1.2×10
-4
and 9.1×10
-3
are also exhibited. The BER in the experiment is
evaluated from the error-vector magnitude (EVM) of the
processed constellations. It can be seen that the BER
performance for all the channels is below the HD-FEC limit
threshold of 3.8×10
-3
with 7% overhead FEC code [22], and the
power penalty reaching the FEC performance between the best
(400 and 425 GHz) and the worst (350 and 475 GHz) channels
is around 3 dB. This penalty can be explained by the un-even
frequency response of the whole THz link shown in Fig. 3(a),
which complies well with the BER performance observation.
When harmonic spurs exist, the measured BER results of 6
channels are shown in Fig. 3(c). Here the electronic spurs
originating from the 4-time frequency multiplier are also
displayed. It can be seen that the BER performance trends of
these 6 channels also complies with the frequency response of
THz link in Fig. 3(a). Besides that, it is noted that the penalties
between those in Fig. 3(b) and (c) at the BER of 3.8×10
-3
are
about 2 dB for the 350 GHz channel, 1.5 dB for 375 GHz,
0.5 dB for 400 GHz, 1 dB for 425 GHz, 1.5 dB for 450 GHz
and 2 dB for 475 GHz, respectively, as shown in Fig. 3(d). In
addition, we measure the spur suppression ratio (SSR) for all
the channels to investigate the interference of spurs. Here the
SSR is evaluated by measuring the down-converted IF signals
without modulation, and the results are displayed in Fig. 3(d).
We can see from Fig. 3(d) that when the SSR is higher than
30 dB, the induced penalty is less than 1 dB, but the penalty
increases to 2 dB when the SSR level decreases to 15 dB. As an
illustration, the measured BER performance and electrical
spectrum in the IF region of the 425 GHz channel are displayed
in Fig. 3(d), for analyzing the BER penalty and the SSR.
IV. CONCLUSION
We propose and experimentally demonstrate a 6-channel THz
wireless communication system in the 350-475 GHz band with
a link capacity of up to 120 Gbit/s. The performance of
Schottky mixer-based THz receiver for detecting multi-channel
THz signals is also investigated. The experimental results
validate that the existence of harmonic spurs in the receiver
with a SSR less than 20 dB within the demodulation bandwidth
will significantly affect the overall system performance, and
suggests more than 30 dB SSR for obtaining less than 1 dB
interference. This work provides the support of further
development of high-speed THz wireless communication
systems.
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![Fig. 1 The increase of data rates for wireless, nomadic and wireline communication technologies against time [1].](/figures/fig-1-the-increase-of-data-rates-for-wireless-nomadic-and-32gfght7.webp)
